The average daily sunspot numbers sank more than 28 points this week (nearly 26 percent) to 81.9, while the average daily solar flux declined more than 16 points to 111.6. This is for the seven days of May 24-30, and is compared to the previous seven days, May 17-23. Sunspot numbers for May 24-30 were 96, 86, 70, 83, 87, 73 and 78, with a mean of 81.9. The 10.7 cm flux was 115.5, 117.2, 110, 110.7, 110.3, 106.3 and 111, with a mean of 111.6. The estimated planetary A indices were 8, 6, 4, 4, 6, 6 and 6, with a mean of 5.7. The estimated mid-latitude A indices were 11, 8, 5, 3, 3, 5 and 5, with a mean of 5.7.

No new sunspots appeared over the five days of May 26-30, but on May 31, three new sunspot groups appeared. One day earlier the sunspot number was 78, while the total relative sunspot area was 400. On May 31, those numbers dropped to 73 and 330, even with the new spots. On May 30, sunspot groups 1486, 1488, 1490 and 1492 were in place. But by May 31, groups 1486 and 1488 disappeared, and new sunspot groups 1493, 1494 and 1495 emerged. The total area of all visible spots was less than the day before.

You can track the fading of sunspot groups and the emergence of new ones here. The dates shown are always for the new UTC day, the one following the day for which the data is shown. So the report marked June 1 is actually displaying data gathered on May 31. In this report, the area for each sunspot group is listed, and this is expressed in millionths of a solar hemisphere. The total sunspot area for the day, along with the sunspot number, is displayed along with solar flux resolved to whole numbers here.

The short term outlook is revised from the one given in yesterday’s ARRL Letter, with the solar flux peaking around 125 on June 2-7, hitting a minimum of 105 on June 27-28, and then peaking at 120 on July 1-12. The latest prediction has solar flux at 120 on June 1, 125 on June 2-7, 120 on June 8-15, 115 on June 16-18, 110 on June 19-26, and 105, 105, 110 and 115 on June 27-30, and then 120 on July 1-12. The predicted planetary A index is 5 on June 1-2, then 10, 15, 18, 15 and 10 on June 3-7, 5 on June 8-11, 8 on June 12-13, 5 on June 14-17, and 15, 12 and 8 on June 18-20, and 5 on June 21-25.

With the start of a new month, we can revisit our moving averages for sunspot numbers. We’ve been observing a three-month moving average, based on whole calendar months, so the latest average is for March, April and May 2012. About a month ago, we were reporting for the months of February, March and April. It is a simple arithmetic (accent on the third, rather than second syllable) average, with a total of all sunspot numbers divided by the number of days in the period.

So over September 1-November 30, 2011, there were 91 days and the sum of all daily sunspot numbers for that period was 10,808, yielding an average daily sunspot number of 118.8 when the sunspot number total is divided by the number of days. The next period covered October 1-December 31, 2011, with a total sum of all daily sunspot numbers of 10,913 divided by 92 days, giving us an average daily sunspot number of 118.6. That period in fall 2011 had the highest sunspot numbers seen so far in Solar Cycle 24.

You might want read this article on Ground Level Enhancements, or GLE, a rare event in which a sudden rise in neutrons coincides with an energetic solar flare or coronal mass ejection. A GLE detected on May 17 was the first GLE of the current solar cycle.

Dave Blood, W1AFC, of Oberlin, Ohio, had some comments on WA8MEA’s recent query on LUF and MUF. Dave wrote: “MUF is affected by solar UV light mostly and therefore is governed by the daily solar flux levels. The LUF, as mentioned, is set by daytime absorption in the D-region at about 100 km heights (hence by the number of radio wave passes through it, up and down), and the number of ionospheric hops supported by the F-region at 300 km heights. The absorption is altered by the instantaneous X-ray radiation from the sun and this can vary up and down highly during solar flare activity. Radio ‘blackout’ conditions occur when the absorption is so high that the LUF exceeds MUF for any propagation path distance for long periods from 20 minutes to several hours.” Dave, who has been a ham for more than 60 years, worked for Raytheon for several decades, and in 1960-1962, on over-the-horizon radar

Rick McCurdy, WA1GTP, is in FN31si in Ivoryton, Connecticut. He wrote: “On May 23 at 1550, I received RADAR on 432.100, first time in years, heading was west-southwest from (Essex) New London/New Haven, Connecticut area. There were very few openings on 6 meters, but on April 30, I worked short E-skip to VE2DLC in FN58rk, a distance of 527 miles. There were also Florida openings May 24, not much else.”

We received another geomagnetic forecast from F.K. Janda, OK1HH. He predicts quiet to active conditions on June 1, quiet on June 2, quiet to unsettled June 3-4, quiet to active June 5, active June 6-7, quiet to active June 8-9, quiet to unsettled June 10, quiet June 11-12, quiet to active June 13, mostly quiet June 14, quiet to unsettled June 15-16, quiet to active June 17, mostly quiet June 18, active June 19, and quiet to unsettled June 20-21.

All times listed are UTC, unless otherwise noted.

Amateur solar observer Tad Cook, K7RA, of Seattle, Washington, provides this weekly report on solar conditions and propagation. This report also is available via W1AW every Friday, and an abbreviated version appears each Thursday in The ARRL Letter. You can find a guide to articles and programs concerning propagation here. Check here and here for a detailed explanation of the numbers used in this bulletin. An archive of past propagation bulletins can be found here. You can find monthly propagation charts between four USA regions and 12 overseas locations here. Readers may contact the author via e-mail.